ACRN Kernel Parameters

Generic kernel parameters

A number of kernel parameters control the behavior of ACRN-based systems. Some are applicable to the Service OS (SOS) kernel, others to the User OS (UOS) kernel, and some are applicable to both.

This section focuses on generic parameters from the Linux kernel which are relevant for configuring or debugging ACRN-based systems.

Parameter Used in SOS or UOS Description Usage example
module_blacklist SOS A comma-separated list of modules that should not be loaded. Useful to debug or work around issues related to specific modules.
no_timer_check SOS,UOS Disables the code which tests for broken timer IRQ sources.
console SOS,UOS

Output console device and options.

Use the virtual console device <n>.

Use the specified serial port and options. Default options are 9600n8 meaning 9600 baud, no parity, 8 bits. Options are of the form bbbbpnf, where:

bbbb is baud rate, for example 9600;
p is parity, one of n, o, or e (for none, odd, or even),
n is number of bits (typically 8),
f is flow control (r for RTS, or left blank)
Use the hypervisor console device <n>. (This is for both Xen and PowerPC hypervisors.)
loglevel SOS

All Kernel messages with a loglevel less than the console loglevel will be printed to the console. The loglevel can also be changed with klogd or other programs. The loglevels are defined as follows:

loglevel value Definition
0 (KERN_EMERG) system is unusable
1 (KERN_ALERT) action must be taken immediately
2 (KERN_CRIT) critical conditions
3 (KERN_ERR) error conditions
4 (KERN_WARNING) warning conditions
5 (KERN_NOTICE) normal but significant condition
6 (KERN_INFO) informational
7 (KERN_DEBUG) debug-level messages
ignore_loglevel UOS Ignoring loglevel setting will print all kernel messages to the console. Useful for debugging. We also add it as printk module parameter, so users could change it dynamically, usually by changing /sys/module/printk/parameters/ignore_loglevel.
log_buf_len UOS Sets the size of the printk ring buffer, in bytes. n must be a power of two and greater than the minimal size. The minimal size is defined by LOG_BUF_SHIFT kernel config parameter. There is also CONFIG_LOG_CPU_MAX_BUF_SHIFT config parameter that allows to increase the default size depending on the number of CPUs. See init/Kconfig for more details.”
consoleblank SOS,UOS The console blank (screen saver) timeout in seconds. Defaults to 600 (10 minutes). A value of 0 disables the blank timer.
rootwait SOS,UOS Wait (indefinitely) for root device to show up. Useful for devices that are detected asynchronously (e.g. USB and MMC devices).
root SOS,UOS

Define the root filesystem

represents the device number of the partition - device number of disk plus the partition number
same as above, this form is used when disk name of the partitioned disk ends with a digit. To separate disk name and partition slot, a ‘p’ is inserted.
representing the unique id of a partition if the partition table provides it. The UUID may be either an EFI/GPT UUID, or refer to an MSDOS partition using the format SSSSSSSS-PP, where SSSSSSSS is a zero-filled hexadecimal representation of the 32-bit “NT disk signature”, and PP is a zero-filled hexadecimal representation of the 1-based partition number.
rw SOS,UOS Mount root device read-write on boot
tsc UOS

Disable clocksource stability checks for TSC.

Format: <string>, where the only supported value is:

Mark TSC clocksource as reliable, and disables clocksource verification at runtime, and the stability checks done at bootup. Used to enable high-resolution timer mode on older hardware, and in virtualized environments.
cma SOS Sets the size of the kernel global memory area for contiguous memory allocations, and optionally the placement constraint by the physical address range of memory allocations. A value of 0 disables CMA altogether. For more information, see include/linux/dma-contiguous.
hvlog SOS Reserve memory for the ACRN hypervisor log. The reserved space should not overlap any other blocks (e.g. hypervisor’s reserved space).
memmap SOS

Mark specific memory as reserved.

Region of memory to be reserved is from ss to ss+nn, using K, M, and G representing Kilobytes, Megabytes, and Gigabytes, respectively.
ramoops.mem_address ramoops.mem_size ramoops.console_size SOS Ramoops is an oops/panic logger that writes its logs to RAM before the system crashes. Ramoops uses a predefined memory area to store the dump. See Linux Kernel Ramoops oops/panic logger for details.
reboot_panic SOS

Reboot in case of panic

The comma-delimited parameters are:

w (warm), s (soft), c (cold), or g (gpio)
b (bios), a (acpi), k (kbd), t (triple), e (efi), or p (pci)
s### (smp, and processor number to be used for rebooting)
f (force), or not specified.
maxcpus UOS

Maximum number of processors that an SMP kernel will bring up during bootup.

maxcpus=n where n >= 0 limits the kernel to bring up n processors during system bootup. Giving n=0 is a special case, equivalent to nosmp,which also disables the I/O APIC.

After bootup, you can bring up additional plugged CPUs by executing

echo 1 > /sys/devices/system/cpu/cpuX/online

nohpet UOS Don’t use the HPET timer
intel_iommu UOS

Intel IOMMU driver (DMAR) option

Enable intel iommu driver.
Disable intel iommu driver.
By default, gfx is mapped as normal device. If a gfx device has a dedicated DMAR unit, the DMAR unit is bypassed by not enabling DMAR with this option. In this case, gfx device will use physical address for DMA.

Intel GVT-g (AcrnGT) Parameters

This table gives an overview of all the Intel GVT-g parameters that are available to tweak the behavior of the graphics sharing (Intel GVT-g, aka AcrnGT) capabilities in ACRN. The GVT-g-kernel-options section below has more details on a few select parameters.

Parameter Used in SOS or UOS Description Usage example
i915.enable_gvt SOS Enable Intel GVT-g graphics virtualization support in the host
i915.enable_pvmmio SOS, UOS Control Para-Virtualized MMIO (PVMMIO). It batches sequential MMIO writes into a shared buffer between the SOS and UOS
i915.gvt_workload_priority SOS Define the priority level of UOS graphics workloads
i915.enable_initial_modeset SOS On MRB, value must be 1. On NUC or UP2 boards, value must be 0. See i915.enable_initial_modeset.
i915.nuclear_pageflip SOS,UOS Force enable atomic functionality on platforms that don’t have full support yet.
i915.avail_planes_per_pipe SOS See i915.avail_planes_per_pipe and i915.domain_plane_owners.
i915.domain_plane_owners SOS See i915.avail_planes_per_pipe and i915.domain_plane_owners.
i915.domain_scaler_owner SOS See i915.domain_scaler_owner
i915.enable_guc SOS Enable GuC load for HuC load.
i915.avail_planes_per_pipe UOS See i915.avail_planes_per_pipe and i915.domain_plane_owners.
i915.enable_guc UOS Disable GuC
i915.enable_hangcheck UOS Disable check GPU activity for detecting hangs.
i915.enable_fbc UOS Enable frame buffer compression for power savings

GVT-g (AcrnGT) Kernel Options details

This section provides additional information and details on the kernel command line options that are related to AcrnGT.


This option enables support for Intel GVT-g graphics virtualization support in the host. By default, it’s not enabled, so we need to add i915.enable_gvt=1 in the SOS kernel command line. This is a Service OS only parameter, and cannot be enabled in the User OS.


We introduce the feature named Para-Virtualized MMIO (PVMMIO) to improve graphics performance of the GVT-g guest. This feature batches sequential MMIO writes into a shared buffer between the Service OS and User OS, and then submits a para-virtualized command to notify to GVT-g in Service OS. This effectively reduces the trap numbers of MMIO operations and improves overall graphics performance.

The i915.enable_pvmmio option controls the optimization levels of the PVMMIO feature: each bit represents a sub-feature of the optimization. By default, all sub-features of PVMMIO are enabled. They can also be selectively enabled or disabled..

The PVMMIO optimization levels are:

  • PVMMIO_ELSP_SUBMIT = 0x1 - Batch submission of the guest graphics workloads
  • PVMMIO_PLANE_UPDATE = 0x2 - Batch plane register update operations
  • PVMMIO_PLANE_WM_UPDATE = 0x4 - Batch watermark registers update operations
  • PVMMIO_MASTER_IRQ = 0x8 - Batch IRQ related registers
  • PVMMIO_PPGTT_UPDATE = 0x10 - Use PVMMIO method to update the PPGTT table of guest.


This parameter works in both the Service OS and User OS, but changes to one will affect the other. For example, if either SOS or UOS disables the PVMMIO_PPGTT_UPDATE feature, this optimization will be disabled for both.


AcrnGT supports Prioritized Rendering as described in the Prioritized Rendering and Preemption high-level design. This configuration option controls the priority level of GVT-g guests. Priority levels range from -1023 to 1023.

The default priority is zero, the same priority as the Service OS. If the level is less than zero, the guest’s priority will be lower than the Service OS, so graphics preemption will work and the prioritized rendering feature will be enabled. If the level is greater than zero, UOS graphics workloads will preempt most of the SOS graphics workloads, except for display updating related workloads that use a default highest priority (1023).

Currently, all UOSes share the same priority. This is a Service OS only parameters, and does not work in the User OS.


At time, kernel graphics must be initialized with a valid display configuration with full display pipeline programming in place before the user space is initialized and without a fbdev & fb console.

When i915.enable_initial_modeset=1, the FBDEV of i915 will not be initialized, so users would not be able to see the fb console on screen. If there is no graphics UI running by default, users will see black screens displayed.

When i915.enable_initial_modeset=0 in SOS, the plane restriction (also known as plane-based domain ownership) feature will be disabled. (See the next section and Plane-Based Domain Ownership in the ACRN GVT-g High Level Design for more information about this feature.)

In the current configuration, we will set i915.enable_initial_modeset=1 in SOS and i915.enable_initial_modeset=0 in UOS.

This parameter is not used on UEFI platforms.

i915.avail_planes_per_pipe and i915.domain_plane_owners

Both Service OS and User OS are provided a set of HW planes where they can display their contents. Since each domain provides its content, there is no need for any extra composition to be done through SOS. i915.avail_planes_per_pipe and i915.domain_plane_owners work together to provide the plane restriction (or plan-based domain ownership) feature.

  • i915.domain_plane_owners

    On Intel’s display hardware, each pipeline contains several planes, which are blended together by their Z-order and rendered to the display monitors. In AcrnGT, we can control each planes’ ownership so that the domains can display contents on the planes they own.

    The i915.domain_plane_owners parameter controls the ownership of all the planes in the system, as shown in Figure 27. Each 4-bit nibble identifies the domain id owner for that plane and a group of 4 nibbles represents a pipe. This is a Service OS only configuration and cannot be modified at runtime. Domain ID 0x0 is for the Service OS, the User OS use domain IDs from 0x1 to 0xF.


    Figure 27 i915.domain_plane_owners

    For example, if we set i915.domain_plane_owners=0x010001101110, the plane ownership will be as shown in Figure 28 - SOS (green) owns plane 1A, 1B, 4B, 1C, and 2C, and UOS #1 owns plane 2A, 3A, 4A, 2B, 3B and 3C.


    Figure 28 i915.domain_plane_owners example

    Some other examples:

    • i915.domain_plane_owners=0x022211110000 - SOS (0x0) owns planes on pipe A; UOS #1 (0x1) owns all planes on pipe B; and UOS #2 (0x2) owns all planes on pipe C (since, in the representation in Figure 27 above, there are only 3 planes attached to pipe C).
    • i915.domain_plane_owners=0x000001110000 - SOS owns all planes on pipe A and pipe C; UOS #1 owns plane 1, 2 and 3 on pipe B. Plane 4 on pipe B is owned by the SOS so that if it wants to display notice message, it can display on top of the UOS.
  • i915.avail_planes_per_pipe

    Option i915.avail_planes_per_pipe is a bitmask (shown in Figure 29) that tells the i915 driver which planes are available and can be exposed to the compositor. This is a parameter that must to be set in each domain. If i915.avail_planes_per_pipe=0, the plane restriction feature is disabled.


    Figure 29 i915.avail_planes_per_pipe

    For example, if we set i915.avail_planes_per_pipe=0x030901 in SOS and i915.avail_planes_per_pipe=0x04060E in UOS, the planes will be as shown in Figure 30 and Figure 30:


    Figure 30 SOS i915.avail_planes_per_pipe


    Figure 31 UOS i915.avail_planes_per_pipe

    i915.avail_planes_per_pipe controls the view of planes from i915 drivers inside of every domain, and i915.domain_plane_owners is the global arbiter controlling which domain can present its content onto the real hardware. Generally, they are aligned. For example, we can set i915.domain_plane_owners= 0x011111110000, i915.avail_planes_per_pipe=0x00000F in SOS, and i915.avail_planes_per_pipe=0x070F00 in domain 1, so every domain will only flip on the planes they owns.

    However, we don’t force alignment: avail_planes_per_pipe might not be aligned with the setting of domain_plane_owners. Consider this example: i915.domain_plane_owners=0x011111110000, i915.avail_planes_per_pipe=0x01010F in SOS and i915.avail_planes_per_pipe=0x070F00 in domain 1. With this configuration, SOS will be able to render on plane 1B and plane 1C, however, the content of plane 1B and plane 1C will not be flipped onto the real hardware.


On each Intel GPU display pipeline, there are several plane scalers to zoom in/out the planes. For example, if a 720p video is played full-screen on a 1080p display monitor, the kernel driver will use a scaler to zoom in the video plane to a 1080p image and present it onto a display pipeline. (Refer to “Intel Open Source Graphics PRM Vol 7: display” for the details.)

On Broxton platforms, Pipe A and Pipe B each have two plane scalers, and Pipe C has one plane scaler. To support the plane scaling in AcrnGT guest OS, we introduced the parameter i915.domain_scaler_owner, to assign a specific scaler to the target guest OS.

As with the parameter i915.domain_plane_owners, each nibble of i915.domain_scaler_owner represents the domain id that owns the scaler; every nibble (4 bits) represents a scaler and every group of 2 nibbles represents a pipe. This is a Service OS only configuration and cannot be modified at runtime. Domain ID 0x0 is for the Service OS, the User OS use domain IDs from 0x1 to 0xF.

For example, if we set i915.domain_scaler_owner=0x021100, the SOS owns scaler 1A, 2A; UOS #1 owns scaler 1B, 2B; and UOS #2 owns scaler 1C.


This parameter enable detection of a GPU hang. When enabled, the i915 will start a timer to check if the workload is completed in a specific time. If not, i915 will treat it as a GPU hang and trigger a GPU reset.

In AcrnGT, the workload in SOS and UOS can be set to different priorities. If SOS is assigned a higher priority than the UOS, the UOS’s workload might not be able to run on the HW on time. This may lead to the guest i915 triggering a hangcheck and lead to a guest GPU reset. This reset is unnecessary so we use i915.enable_hangcheck=0 to disable this timeout check and prevent guest from triggering unnecessary GPU resets.